Conventionally, an obstacle detecting apparatus includes a plurality of ultrasonic sensors. In such an obstacle detecting apparatus, an ECU is connected to the sensors through signal lines extending from the ECU in a star topology. However, with a demand for further increasing the number of sensors, it becomes difficult to provide signal lines in a star topology for the reason of space, weight and the like.
In order to cope with the above problem, an obstacle detecting apparatus for the purpose of reducing the number of lines has been proposed in Japanese Patent Laid-Open Publication No. 2003-279651. According to the obstacle detecting apparatus disclosed in this publication, a plurality of sensors and an ECU are connected to each other through a communication line provided in a bus topology. Through the thus provided communication line, intercommunication therebetween is achieved.
In the case where a communication error occurs, the obstacle detecting apparatus using the communication lines as described above generally notifies a user of the occurrence of a communication error as quickly as possible. If the occurrence of such a communication error is to be detected, for example, the following method is generally used. According to this method, error determination time for determining the occurrence of an error is set. The occurrence of a communication error is determined in the case where the communication is impossible for the error determination time or longer, thereby detecting the communication error.
If the error determination time is long, however, the occurrence of a communication error cannot be reported as quickly as possible because the communication error cannot be detected at the early stage. On the other hand, if the error determination time is short, a communication error is detected even if a state gets back to normal immediately after the occurrence of a temporary error. Therefore, the occurrence of an error is erroneously reported.